This invention relates to torque sensors, more specifically to magnetostrictive torque sensors.
The magnetostrictive effect may be termed as the change of dimensions of a material when exposed to a magnetic field or its inverse effect, i.e. the change in magnetization of a material as a result of external stress. This inverse magnetostrictive effect is sometimes referred to as the magnetoelastic effect, but the term magnetostrictive is used exclusively in the present document. Generally, the magnetostrictive effect is associated with ferromagnetic materials.
U.S. Pat. No. 5,351,555 discloses a non-contact torque sensor of the magnetostrictive type which can be used with rotating shafts. As illustrated in FIG. 16 of the accompanying drawings, the torque sensor comprises a collar 120 fitted tightly onto a shaft 116. The collar 120 is magnetized circumferentially around the shaft as indicated by arrows in the figure. When the shaft 116 is torqued, the torque is transmitted to the collar 120 and induces a helical magnetic field therein. A component of the helical field is sensed by an externally positioned magnetic flux detector 118 from which the magnitude of the torque can be inferred.
Although these designs work well, they have been criticized for several reasons. One problem is that under high torque conditions it is possible that slippage of the collar on the shaft may occur. Another issue is the manufacturing costs associated with making and fitting the collar to the shaft, which have been said to be too high.
WO 99/21150, WO 99/21151 and WO 99/56099 disclose various designs of torque sensor which address the shortcomings of the collar-based designs. In these more recent designs, a portion of the shaft itself is magnetized, thereby allowing a separate magnetized collar to be dispensed with. FIG. 17 of the accompanying drawings illustrates an example of these collarless designs. A shaft 116 has integral portions thereof 122 magnetically polarized in the circumferential direction, i.e. around the shaft, as in the collar-based designs. Multiple polarized regions are preferred, with adjacent domains being oppositely magnetically polarized, as illustrated in the figure in which two such domains are shown. Torquing of the shaft causes a change in the magnetic field external to the shaft, which is measured by a suitable magnetic flux detector 118, similar to the collar-based designs.
One problem common to all these designs is their dependence on the permanent polarization of the collar or shaft. The magnetic polarization is induced during manufacturing, but manufacturing variations cause variations in the polarization strength, which in turn cause variations in sensitivity between different sensors. Although measures are proposed in WO 99/56099 to control this variation, the measures are quite complex. More seriously, all of the above-mentioned designs depend on, and assume, long-term stability of the magnetically polarized part of the sensor. If the magnetic polarization decays, then a given external torque applied to the shaft will result in a lower output from the magnetic flux detector. Periodic recalibration of the sensor will therefore be required if absolute sensitivity is needed. If the decay is more serious, then remagnetisation of the magnetized part of the torque sensor or more likely replacement of the whole sensor will be necessary.
It is therefore an aim of the invention to provide a torque sensor that reduces the prior art dependence on magnetic polarization strength.